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How to prove that if the computer time is changed (by an operator or maybe a higher level of access) the "real" time related to data is not changed?

I imagine that Siemens system is enough secured but in the pharmaceutical sector we need to perform appropriate tests to confirm that.

In this case, it's an air conditionning system (HVAC), the data are currently locally saved.Till nown there is no network infrastructure....but the data (temperature) are manually written by the operator (each hour) during the week.

Hi,Am not sure I understand your question well, But as I understand you have SCADA System with some databse (may be SQL) for storing data (time and temperature) let assume it has the following data structure for the table:

Case - 1: If the table ID (the unique item) of the above table is TIME column and let say the time is changed back to 7/4/2012 09:00 AM and then a new data TIME = 7/4/2012 10:25 AM, TEMPERATURE = 18has to be stored to the table; but note that the table has already a data at TIME = 7/4/2012 10:25 AM. Depending on the program the new data will be replaced by the new one or prevented.

Case - 2: If the table ID of the above table is NO column, and if similar case happens as Case -1 the new data will be added at the end of table as NO = 05, TIME = 7/4/2012 10:25 AM, TEMPERATURE = 18 and you will have similar TIME data at NO = 03 & 05.

In general my answer for you is, Inorder to check the data is changed by new one, you have to see the source program and study the data structure or contact the programmer. The other way is use artistic way for example enter two similar data and observe the stored values.